1. Field of the Invention
This invention relates to an electric power receiving circuit and a responder, including the same, for an automatic vehicle identification system including an interrogator for identifying the responder existing in the service area of the interrogator through transmitting a radio signal.
2. Description of the Prior Art
Responders for an automatic vehicle (mobile unit) identification system including an interrogator for identifying the responder existing in the service area of the interrogator through using radio signals are known. The responder which may be held by a man or mounded on a mobile unit such as an automatic vehicle, existing in or enter the service area of the interrogator returns a radio signal (microwave) from the interrogator with modulation with a data stored therein to identify the responder.
FIG. 18 is a plan view, a partially schematic, of a prior art electric power receiving circuit having a microstrip line antenna included in a prior art responder (not shown). This prior art electric power receiving circuit including a microstrip line antenna comprises a radiation patch 7 having a half wavelength formed on a substrate 9, microstrip lines 5a to 5c formed on the substrate 9, a diode 8a for sending a modulated signal to the microstrip line 5a, and diodes 8b to 8c for rectifying a received radio signals. This strip line antenna features that no special power source such as a battery is required. However, there is a problem that the size of the radiation patch has a half wave length of the interrogation signal, so that the dimension of the responder becomes large.
Moreover, there is a problem that the energy of the microwave transmitted from the interrogator cannot be received by the prior art responder if an attitude of the responder does not agree with a polarizing plane of the microwave.